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3.9.3 Document Merlin domain_separate conflict potential and add an asert

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Luke Parker 2023-03-06 20:16:57 -05:00
parent 7efedb9a91
commit 20a33079f8
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1 changed files with 19 additions and 6 deletions
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25
crypto/transcript/src/merlin.rs
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@ -2,8 +2,18 @@ use core::fmt::{Debug, Formatter};
use crate::Transcript; use crate::Transcript;
/// A wrapper around a Merlin transcript which satisfiees the Transcript API.
///
/// Challenges are fixed to 64 bytes, despite Merlin supporting variable length challenges.
///
/// This implementation is intended to remain in the spirit of Merlin more than it's intended to be
/// in the spirit of the provided DigestTranscript. While DigestTranscript uses flags for each of
/// its different field types, the domain_separate function simply appends a message with a label
/// of "dom-sep", Merlin's preferred domain separation label. Since this could introduce transcript
/// conflicts between a domain separation and a message with a label of "dom-sep", the
/// append_message function uses an assertion to prevent such labels.
#[derive(Clone)] #[derive(Clone)]
pub struct MerlinTranscript(pub merlin::Transcript); pub struct MerlinTranscript(merlin::Transcript);
// Merlin doesn't implement Debug so provide a stub which won't panic // Merlin doesn't implement Debug so provide a stub which won't panic
impl Debug for MerlinTranscript { impl Debug for MerlinTranscript {
fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), core::fmt::Error> { fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), core::fmt::Error> {
@ -12,11 +22,10 @@ impl Debug for MerlinTranscript {
} }
impl Transcript for MerlinTranscript { impl Transcript for MerlinTranscript {
// Uses a challenge length of 64 bytes to support wide reduction on generated scalars // Uses a challenge length of 64 bytes to support wide reduction on commonly used EC scalars
// From a security level standpoint, this should just be 32 bytes // From a security level standpoint (Merlin targets 128-bits), this should just be 32 bytes
// From a Merlin standpoint, this should be variable per call // From a Merlin standpoint, this should be variable per call
// From a practical standpoint, this is a demo file not planned to be used and anything using // From a practical standpoint, this should be practical
// this wrapper should be secure with this setting
type Challenge = [u8; 64]; type Challenge = [u8; 64];
fn new(name: &'static [u8]) -> Self { fn new(name: &'static [u8]) -> Self {
@ -24,10 +33,14 @@ impl Transcript for MerlinTranscript {
} }
fn domain_separate(&mut self, label: &'static [u8]) { fn domain_separate(&mut self, label: &'static [u8]) {
self.append_message(b"dom-sep", label); self.0.append_message(b"dom-sep", label);
} }
fn append_message<M: AsRef<[u8]>>(&mut self, label: &'static [u8], message: M) { fn append_message<M: AsRef<[u8]>>(&mut self, label: &'static [u8], message: M) {
assert!(
label != "dom-sep".as_bytes(),
"\"dom-sep\" is reserved for the domain_separate function",
);
self.0.append_message(label, message.as_ref()); self.0.append_message(label, message.as_ref());
} }